Is there a website that allow me to upload my GPS file from UTM to lat and long to produce a new output file ? I am looking for one to run a batch a list of UTM or LL to converstion them in either way. Thanks.

I am aware of know how to do perfrom this in ArcMap, but what if you wanted them in a hurry like doing that for ArcGIS Explorer online.

I'm not aware of a website application, but you might take a look at Corpscon, a free application from the US Army Corps of Engineers. It provides coordinate conversion functions. It will convert text files containing coordinate information from one coordinate system to another. If your GPS data is in a GIS data format you would need to export it to a coordinate file of some sort.